1. Field of the Invention
The present invention relates to a controller for oil wells and a method for determining the presence of oil in the well and accordingly controlling the pumping operation. In particular, the present invention is directed to a thermal dispersion probe for detecting the flow rate of oil extracted with an oil pump jack and related controller for regulating the pump starting and stopping based on measurements given by the probe.
2. Description of Related Art
In oil-well pump recovery operations, strategically anchored pumping jacks lift oil in discrete quantities to the surface through an originating underground well-pocket. These pumping jacks are powered by either electrical or natural gas motors, and the fluid produced consists generally of a mixture of crude oil, natural gas, dirt, wax, grit, and water. Upon reaching the surface, this combination is passed on to a separation system where oil in the fluid is released and temporarily stored.
Based on geological formation and well-type, the maximum attainable production rate depends on the migration of crude oil from underground porous rock and sand into the well-pocket. Typically, in mature wells, the capacity of the oil pumping jack exceeds the capacity of the surrounding formation to refill the well-pocket. Hence, there exists a requirement, especially for older wells, to control pumping action in response to changing rates of oil flow to the surface. For example, pumping action exceeding proportional oil recovery capacity can lead to severe pipe and pump jack degradation, while promoting surplus energy consumption exceeding required cost.
Many approaches to circumvent these problems have been proposed, all generally based on establishing correlations between the rate of oil flow and the pump jack operation. However, the search for an adequate solution has proven historically difficult.
For example, timers designed to allow pumping action for selected lengths of time have been installed at well sites. Periods are then adjusted in an attempt to match the flow characteristics of the well. However, while conceptually simple, the approach can neither accommodate the typical wide variations in oil production on a daily basis nor adjust itself to longer term changes in declining wells.
In general, oil-well pump control instrumentation may be classified according to the techniques used to signal change in fluid production. The most common include: flow ram change detection, polished rod load, motor torque or vibration sensing, and bottom hold pressure detection.
In flow rate change detection, sensing elements placed into the flow line signal reductions in fluid production over assigned timing intervals. If production falls below a given threshold during successive intervals pump jack operation is halted. The sensor is calibrated to sense the reduction in pumping rate over a preselected period of time.
For example, a sensor describing a mechanically actuated switch is disclosed in U.S. Pat. No. 3,936,231 (Douglas) issued Feb. 3, 1976. Here, oil emerging from the well is directed through a valve assembly. Whenever pumping action causes fluid to surge, the valve, activated in response, closes a magnetic switch thereby acknowledging that oil is present.
A problem inherent for these types of sensors is that unrefined oil pumped to the surface normally contains impurities such as dirt and sand, plus dissolved gas. Approaches based on mechanical designs (for example, paddle wheels) to measure the flow tend to become susceptible to expanding gas, leading to skewed flow results, while impurities in the oil over time tend to clog moving parts.
Other types of sensors have been based upon the properties of thermal dispersion. Such sensors use a heater and temperature measuring devices as for example, thermistors. European Patent Application 440155 A2 (Kufio et al.), published Jan. 29, 1992; and U.S. Pat. No. 4,392,782 (Kuehn et al.) issued Jul. 12, 1983 disclose such sensors. Generally, the heater embodies a heated resistive coil, as shown in Canadian Patent No. 1,044,037 (Schopp) issued Dec. 12, 1978, and Canadian Patent Applications No. 2,047,735 (Grimson et al.) published Aug. 19, 1990 and No. 2,018,272 (Micker) published Dec. 8, 1990. Alternative heat sources are also used, as microwave radiation (Canadian Patent Application No. 2,011,439 (Goldberg) published Sep. 6, 1990), and transistors (Canadian Patent No. 1,043,124 (Olmstead) issued on Nov. 28, 1978).
However, in order to obtain a reliable and accurate measurement, the power dissipation of the heating source should be maintained constant. A constant power dissipation is obtained with a constant power level applied to the heating element.
Another important property of the oil flow controller is that expanding gas flowing over the thermistors tends to not signal higher flow rates (as compared to paddle-wheel and other mechanical approaches) due to the lower density of the gas in comparison to oil.